The present invention relates to an improvement in a method for the formation of a silica-based coating film on the surface of a substrate. More particularly, the invention relates to an improvement in the method for the formation, on the surface of a substrate, of a silica-based coating film having a relatively large thickness and high uniformity as well as denseness and capable of exhibiting excellent adhesion to the substrate surface and resistance against crack formation so as to be useful, for example, as an interlayer insulating film, planarizing film, intermediate film, phase-shifter material, protective film, orientation film and the like in the manufacture of semiconductor devices and liquid crystal display devices.
In the manufacturing processes of semiconductor devices, liquid crystal display devices and the like in the prior art, in which planarizing films, interlayer insulating films, protective films, orientation films, intermediate films in a multilayered resist and phase-shifter materials are used, the most widely used material for the films is silica. As a method for the formation of such a silica-based coating film on the substrate surface, the vapor-phase growing method and wet-process coating method are usually undertaken.
The former method of the vapor-phase growing, which is a method in which silica is deposited from the vapor phase on the surface of a substrate by using a special apparatus, is disadvantageous in respect of the use of a very expensive apparatus and a relatively low productivity of the process in the industrial manufacture of semiconductor devices and liquid crystal display devices.
On the other hand, the latter method of wet-process coating, in which a substrate surface is coated with a coating solution in an organic solvent containing a hydrolyzate of an alkoxy silane compound followed by a baking treatment to convert the hydrolyzate of the silane compound into silica, is advantageous and widely employed in the electronics industry because no expensive apparatus is required and the productivity of the process is considerably high.
One of the typical coating compositions or coating solutions for the formation of a silica-based coating film used in the above mentioned coating method is prepared by dissolving, for example, a tetraalkoxy silane compound or a monoalkyl trialkoxy silane compound having a lower alkyl group in the molecule in an organic solvent which is mainly an alcohol to effect hydrolysis of the silane compound in the solution (see Japanese Patent Kokai 63-241076). The coating solutions of this type are fully suitable for the manufacture of semiconductor devices of a high degree of integration such as the 4-mega DRAMs and 16-mega DRAMs prepared by fine patterning of sub-micron order of fineness with a line width of about 0.8 .mu.m or half-micron order of fineness with a line width of about 0.5 .mu.m as the major current heretofore. Along with the recent trend toward higher and higher degree of integration for 64-mega DRAMs and 1-giga DRAMs in the technology of semiconductor devices, a still finer patterning with a line width of 0.35 .mu.m or smaller is required, for which the above described coating solutions can no longer meet the requirements in recent years.
Taking the case of formation of a planarizing film for ultrafine patterning by using the above mentioned coating solution as an example, several detrimental phenomena are caused by the remaining gases produced in the course of drying and heat treatment of the hydrolyzate of the alkoxy silane compound, such as methane and the like as the decomposition products of the alkyl groups in the alkoxy silane compound and water vapor, resulting in a decrease in the adhesive bonding strength of the upper coating layer to the planarizing film or corrosion of the metallic wiring layer. When the planarizing treatment involves a step of etch-back, the thickness of the silica-based coating film on the interlayer insulating film is large in the densely patterned areas and small in the sparsely patterned areas with a great difference in the film thickness between the densely and sparsely patterned areas. This phenomenon results in a trouble that, when an etch-back treatment is undertaken, the interlayer insulating film is also removed by etching to expose the metallic wiring layer bare in the sparsely patterned areas. When the metallic wiring layer is exposed on the surface, shortcircuiting may eventually be caused between the wirings so that reliability of the electronic device is greatly decreased. Further, an ultrafine pattern of 0.35 .mu.m line width or finer cannot be completely filled up resulting in the appearance of so-called "blowholes".
On the other hand, polysilazane solutions are known as a coating solution for the formation of a silica-based coating film. As compared with the coating solutions of a hydrolysis product of an alkoxy silane compound, the coating method using a polysilazane solution is highlighted in recent years because of the absence of the above described problems such as gas evolution, film thickness difference between densely and sparsely patterned areas, absence of "blowholes" and so on and possibility of obtaining a silica-based coating film having a large thickness, denseness and good adhesion to the substrate surface as well as excellent resistance against crack formation. Polysilazane solutions, however, are practically disadvantageous in handling because the solution is extremely unstable to cause early gelation when exposed to atmospheric air.
Several improvements have been proposed heretofore in the coating method by using a polysilazane solution including a method in which coating of the substrate surface with a polysilazane solution is followed by drying and a baking treatment is undertaken in an atmosphere of oxygen or water vapor as disclosed in Japanese Patent Kokai 5-121572, a method in which curing of the coating film is effected by heating or ultraviolet irradiation as disclosed in Japanese Patent Kokai 5-105486 and 5-119307 and a method in which curing of the coating film is effected as disclosed in Japanese Patent Kokai 5-243223. A problem in these prior art methods, however, is that some of the Si-N linkages and Si-H linkages remain undecomposed in the coating film after curing unavoidably to decrease the denseness and uniformity of the coating film. The method of the baking treatment in an atmosphere of water vapor has another problem that diffusion of the water molecules occurs to the interlayer insulating film formed on the metallic wiring layer to cause corrosion of the metallic wiring layer resulting in a decrease in the reliability of the semiconductor device as the final product.